The oxidative dyeing of hair is one of the most extensively used methods to color hair. In this process, oxidative hair coloring precursors are used in combination with an oxidizing agent, commonly a peroxy oxidizing agent. The oxidative hair coloring precursors are commonly classified into two subclasses; i.e. primary intermediates and couplers. The precursors are generally small molecules capable of diffusing into hair. Another common feature of these precursors is that they mostly contain groups such as amino groups. The presence of these groups is essential to enable the chemistry associated with the oxidative hair dyeing.
While on one hand these amino groups need to be present in the chemical skeleton of the hair coloring precursors to allow the oxidative hair dyeing chemistry to occur, on the other hand one of the drawbacks is that the hair coloring precursors are relatively unstable and degrade over time, especially when exposed under light, humid environment and air. The main degradation products seem to be the oxidized derivatives of these precursors and dimeres thereof.
As known in the hair dyeing field, the oxidative hair dyeing products may be sold in a kit comprising different components to be mixed by the consumer or salon stylist to obtain the desired hair dyeing composition. This kit comprises a so-called tint component comprising the hair coloring dye precursors along with the so-called developer component comprising an oxidizing agent. From the time the hair coloring precursors is manufactured, packed, transported and stored until the material is introduced into the hair dye formulation a long period can lapse. During this period, it has been shown that degradation of the hair dyeing precursors can occur. It has been noticed that the longer these precursors were stored, the higher was the amount of degradation products, in particular oxidized derivatives of these precursors and dimers thereof.
A way to decrease the rate of degradation of the hair dye coloring precursors is to transform them into their corresponding salts in order to maintain these compounds in a reasonably stable stage up to the moment they will be formulated in the tint component. Although more stability on storage can be reached by using the salts of the hair coloring precursors, these salts derivatives may, however, have an impact on the hair dyeing performance.
4,5-diaminopyrazole derivatives are commonly used in the oxidative hair dyeing area due to their good performance as primary intermediates. They give varied shades when mixed with couplers in an oxidative hair dyeing medium.
U.S. Pat. No. 5,663,366 discloses a process for producing 4,5-diaminopyrazole derivatives of the general formula (A):

where R1 and R2 independently designate hydrogen, a C1- to C6-alkyl radical or a C2- to C4-hydroxyalkyl radical, which can be used as dye precursors, for example for hair dyes. The examples describe the process leading to the synthesis of 4,5-diaminopyrazole hydrosulfate, hydrosulfate hydrate and dihydrochloride salts derivatives.
U.S. Pat. No. 7,018,426 discloses an agent for the oxidative dyeing of keratin fibers, wherein it contains (a) at least one 4,5-diaminopyrazole derivative of formula (B), (C), (D) or its salts with organic or inorganic acids as well as (b) at least one resorcinol derivative of the general formula (E), wherein R1 and R2 independently of one another represent hydrogen, a linear or branched C1-C6 alkyl group, an optionally substituted phenyl group or a linear or branched C2-C4-hydroxyalkyl group.

The examples disclose sulfate, hemisulfate, tetrachloride and dihydrochloride salts of 4,5-diaminopyrazole compounds but not 1-Hexyl-1H-pyrazole-4,5-diamine hemisulfate.
There is a constant need to find primary intermediates which provide good hair dyeing performance as well as good stability during storage, especially in terms of stability over exposure under light, humidity and air.
The invention relates to a novel primary intermediate 1-hexyl-1H-pyrazole-4,5-diamine hemisulfate, which was found to combine a very good stability profile during storage as well as providing the desired colour shade with excellent color intensity.